Variable inductors



March 30, 1965 w. BROOKS VARIABLE INDUCTORS Filed Aug, 11 1961 2 sheets-S 1 INVENTOR, WILLIAM BROOKS gent March 30, 1965 w. BROOKS VARIABLE INDUCTORS 2 Sheets-Sheet 2 Filed Aug. 11, 1961 INVENTOR. WILLIAM BROOKS Agent United States Patent 3,176,218 VARIABLE ENDUCTORS William Brooks, Sunnyvale, Calif., assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Aug. 11, 1961, Ser. No. 130,845 Claims. (Cl. 323-85) This invention relates to variable inductors and more particularly to such inductors wherein physical movement of the core is not necessary to cause variation in inductance.

In the prior art variable inductors of which I have knowledge it is necessary to make a physical connection with the core of the inductor in order to adjust it. This is particularly disadvantageous when the inductor is being used in an ultrahigh-frequency circuit, because the physical connection with the core causes a variation in inductance without even adjusting the core. That disadvantage has necessitated the use of variable capacitors with a fixed inductor in resonance circuits and such circuit hasa very low Q.

Therefore it is an object of my invention to provide a variable inductor in which inductance may be varied without direct physical contact with the inductor or its core.

Another object of my invention is to provide a variable inductor having a very high Q.

A further object is to provide a variable inductor in which the inductance varies in proportion to the amount of flight impinging on the inductor.

These and other objects will be more apparent after referring to the following specification and attached drawings in which:

FIGURE 1 is a perspective view of a preferred embodiment of my invention;

FIGURE 2 is a schematic diagram of a circuit utilizing my improved variable inductor;

FIGURE 3 is a perspective view of another embodiment of my invention; and

FIGURE 4 is a cross-sectional view of still another embodiment of my invention.

Referring more particularly to the drawings, reference numeral 2 indicates a core which may be made of any suitable insulative or ceramic material and which may be either hollow or solid. Coated on the exterior surface of core 2 is a strip of photoconductive material 4 which has the characteristic that its resistance to the flow of electrical current decreases as the amount of light impinging thereon increases. Photoconductive strip 4 may be made of anyone of a number of conventional photoconductive substances such as cadmium sulfide or cadmium selenide. The remainder of the exterior surface of core 2 has coated thereon a conductive film 6 which may be one of a number of conventional materials, for example, copper, silver, gold, or titanium metal. Core 2 is suitably mounted on an insulative base 8 which has attached thereto a coil support 10 which is constructed of any suitable insulative material. Supported by coil support 10 is a coil 12 having a plurality of spaced-apart conducting turns 14. The coil 12 is preferably constructed of copper wire and has a diameter and number of turns selected in a well-known manner in accordance with the total inductance desired. In this embodiment of my invention it is necessary that there exist space between adjacent turns 14 so as to permit light to impinge on the strip 4. My improved inductor is mounted in an opaque enclosure 16 which has a lightsource 18 in the interior thereof. Between light-source 18 and photoconductive strip 4 is placed a conventional lens 20 which properly distributes the light from source 18 over the surface of photoconductive strip 4. Lightsource 18 is energized from a battery 22 through a variable ice resistor 24 and adjustment of the latter will vary the light output of the light source 18. FIGURE 2 shows my device connected in a well-known Hartley oscillator which is only one example of the use of my device. In parallel with the coil 12 is a capacitor 26 and the co-action of coil 12 and capacitor 26 determines the frequency of oscillation of the oscillator in a well-known manner. A battery 27 has its negative terminal grounded and its positive terminal connected to an appropriately located tap 28 on coil 12. The lower terminal of capacitor 26 is connected to the base of a transistor 30 through a resistor 32 and a capacitor 34 in parallel. Suitable bias current for the base of transistor 30 is provided by a resistor 36 to ground. The upper terminal of capacitor 26 is connected to the collector of the transistor 30 and is also connected to output terminal 38. The other output terminal 40 is grounded. In a wellknown manner the emitter of transistor 30 is properly biased by resistors 42 and 44 to ground. Resistor 44 is by-passed to ground by a capacitor 46.

The operation of this embodiment of my invention is as follows: with light source 18 completely extinguished the oscillator will generate an alternating current signal whose frequency is determined by the co-action of the coil 12 and capacitor 26. As light source 18 is turned on by adjustment of variable resistor 24, the electrical resistance of the photoconductive strip 4 will decrease and therefore current induced by coil 12 will be permitted to flow through conductive layer 6 and through photoconductive layer 4. As is well-known by those skilled in the art, such current flow will cause the inductance of the coil 12 to increase, thereby causing the frequency of the alternating current signal appearing at terminals 38 and 40 to decrease. Further decrease in the frequency of oscillation is realized as the variable resistor 24 is adjusted to increase the light output of light source 18.

In FIGURE 3 the core 2 has coated on the surface thereof a tapered strip of photoconductive material 4' and the remainder of the exterior surface of core 2 has conductive film 6 coated thereon. The coil 12 has insulated wires such that the coil may be wound directly on the surface of coatings 4' and 6. Extending upwardly from insulative base 8 are two supporting members 43 and 50 in which a shaft 52 is suitably journaled. Attached to shaft 52 is a light source 54 and an opaque shield 56 having a slot 58 therein. Thus as shaft 52 is rotated, a horizontally disposed area of light is caused to move up and down the photoconductive strip 4'.

The operation of this embodiment of my invention is as follows: in a circuit such as that shown in FIGURE 2 incorporating the embodiment shown in FIGURE 3'the frequency of oscillation may be changed by turning shaft 52. Depending on the height of the illuminated area, varying amounts of current will be permittedto flow through photoconductive layer 4'. Thus, the when the lower port-ion of photoconductive strip 4' is illuminated, the inductance of my inductor will be relatively low while when the upper and wider portions of the strip 4 are illuminated the inductance will be relatively high. The device of FIGURE 3 is preferably operated in an opaque enclosure so that external light will not affect the inductance thereof and light source 54 is preferably shielded so that its entire light output occurs through slot 58.

In the embodiments described hereinabove it is necessary that the individual turns 14 of the coil 12 are spaced from one another so as to permit the light from source 18 or 54 to impinge upon the photoconductive layer 4 or 4'. This requirement is eliminated by the embodiment shown in FIGURE 4 wherein the core 2, with photoconductive strip 4 and conductive layer 6, has applied to the exterior thereof a layer of transparent plastic 60 such as is available under the trademark Lucite. On the exterior surface of plastic layer 64 is an opaque insulative coating 62 which may be titanium dioxide. Around the coating 62 may then be wound a coil 12 with adjacent turns touching one another if desired. A variable light source (not shown) is then disposed at the longitudinal end of the device and the inductor and light source are then placed in an opaque enclosure.

The operation of this embodiment of my invention is as follows: as the illumination is varied, varying amounts of light are conducted through plastic layer 60 to the photoconductive strip 4 and the inductance caused to vary as heretofore described. The plastic layer 6d may be thought of as a light pipe which conducts the light from the light source to the photoconductive layer '4. 7

It will be obvious to those skilled in the art that a plurality of my improved variable inductors maybe operated from one shaft. With respect to the embodiment of FIGURE :2, any number of variable resistors may be operated from the same shaft and in the embodiment of FIGURE 3 any number of light sources 54 and the associated equipment may be connected to a common shaft 52. When a plurality of my variable inductors are so utilized it is necessary that each be placed in an opaque enclosure. Although I have shown manual variations of the light intensity impinging on the photoconductive strip 4 it is obvious that there are many ways for varying the intensity of the light impinging On the strip 4.

While several embodiments of my invention have been shown and described it will be obvious that various adaptations and modifications can be made Without departing trom the scope of my invention as defined in the following claims:

I claim as my invention:

1. A variable inductor comprising a core having an elongated insulative body, a strip of photoeonductive material attached to the surface of said body extending substantially from one end of the body to the opposite end and a coating of conductive material on the remainder of the surface of said body, a coil having spaced apart conducting turns around said core, and means for variably illuminating said strip.

2. A variable inductor comprising a. core having an elongated insulative body, a strip of photoco-nductive material attached to the surface of said body extending substantially :from one end of the body to the opposite end, and a coating of conductive material on the remainder of the surface of said body, a coil having spaced apart conducting turns around said core, means for illuminating said strip, and means for controllably varying the output of said illuminating means. A

3. A variable inductor comprising a core having an elongated insulative body, a strip' of photoconductive material attached to the surface of said body extending substantially from one end of the body to the opposite end, and being disposed generally parallel to the longitudinal axis of said body, and a coating of conductive material on the remainder of the surface of said body, a coil having spaced apartconducting'turns around said core, means for illuminating said strip, and means "for controllably varying the output of said illuminating means.

4. A variable inductor comprising a core having an elongated insulative body, a strip of photoconductive material attached to the surface of said body extending substantially from one end of the body to the opposite end, and a coating of conductive material on the remainder of the surface of said body, a coil having spaced apart conducting turns around said core, means for illuminating substantially from one end of the body to the opposite end, a coating of conductive material on the remainder of the surfaceof said body, a "layer of transparent insulative material on said photoconductive material and said conductive coating, and an opaque layer on said transparent layer, a coil having conducting turns around said core, and means for variably illuminating said strip, said illuminating means being disposed at the longitudinal end of said core.

6. A variable inductor comprising a core having an elongated insulative body, a strip of photoconductive m.- terial attached to the surface of said body extending substantially from one end of the body to the opposite end, said strip being generally taper-shaped, and a coating of conductive material on the remainder of the surface of said body, a coil having spaced apart conducting turns around said core, and means for illuminating only a selected longitudinal portion of said photoconductive strip.

7. A variable inductor comprising a core having an elongated insulative body, a strip of photoconductive material attached to the surface or" said body extending substantially from one end of the body to the opposite end thereof, and a coating of conductive materialon the remainder of the surface of said body, a coil having conducting turns around said core, and means for illuminating said strip.

8. A variable inductor comprising a core having an elongated insulative body, a strip of photoconductive material attached to the surface of said body extending substantially from one end of the body to the opposite end thereof, and acoating of conductive material on the remainder of the surrface of said body, a coil having conducting turns around said core, means for illuminating said strip, and means for controlling the output of said illuminating means. 7

9. A variable inductor comprising a core having an elongated insulative body, photoconductive material attached to the surface of said body and extending substantially from one end of the body to the opposite end thereof, a coating of conductive material attached to the remainder of the surface of said body, a coil having conducting turns around said core, and means for illuminating said photoconductive material.

10. A variable inductor comprising a core having an elongated insulative body, a strip of photoconductive material attached to the surface of said body extending from one end of the body to the opposite end thereof,

said strip being of constant width and the longitudinal axis thereof being disposed about parallel to the longitudinal axis of said body, a coating of conductive material at-. tached to the remainder of the surface of said body, a coil having spaced apart conducting turns around said core, and means exterior of said coil for illuminating said strip.

References Cited by the Examiner UNITED STATES PATENTS 2,905,912 9/59 Geiser 33673 4/60 Kramer 33673' 

7. A VARIABLE INDUCTOR COMPRISING A CORE HAVING AN ELONGATED INSULATIVE BODY, A STRIP OF PHOTOCONDUCTIVE MATERIAL ATTACHED TO THE SURFACE OF SAID BODY EXTENDING SUBSTANTIALLY FROM ONE END OF THE BODY TO THE OPPOSITE END THEREOF, AND A COATING OF CONDUCTIVE MATERIAL ON THE REMAINDER OF THE SURFACE OF SAID BODY, A COIL HAVING CONDUCTING TERMS AROUND SAID CORE, AND MEANS FOR ILLUMINATING SAID STRIP. 